The present invention relates to droplet deposition apparatus, in particular inkjet printheads, having a bottom sheet comprising piezo-electric material and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls and by a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material. The present invention also includes methods of manufacture of such apparatus.
Such printheads are known in the artxe2x80x94e.g. from EP-A-0 277 703, EP-A-0 278 590 and EP-A-0 364 136xe2x80x94and employ piezoelectric material actuated in its shear mode to vary the volume of an ink chamber and thereby eject an ink drop. EP-A-0 341 929 describes a method for driving such actuators and makes clear that the actuators each have a capacitive load. As will be generally understood, such a capacitive load makes current and power demands which affect both size and cost of the driving circuitry. Furthermore, it has been recognised by the present inventors that current flowing in the circuitry and electrodes of the printhead itself may also lead to heat generation which in turn will affect the viscosity of the ink: as is the case in many types of inkjet apparatus, variation in ink viscosity will result in a variation in the velocity of the ejected ink droplet which will in turn manifest itself as a droplet placement error in the printed image.
It has been proposed to decrease the capacitive load of a printhead by restricting the use of piezoelectric material in the printhead to those areas where piezoelectric activity is actually requiredxe2x80x94i.e. in the channel walls of the xe2x80x9cactivexe2x80x9d (closed) portions of the channelsxe2x80x94and using material having a lower dielectric coefficient for the remainder of the printhead. This may be achieved in practice by xe2x80x9cletting inxe2x80x9d an insert of piezo-electric material into a lower dielectric base, forming channels that pass through both the insert and the base and depositing electrodes along the length of each channel. Only that part of each channel wall which comprises piezoelectric material will distort in response to an electric field applied via the electrodes, with the remainder of each wall and any associated connection area only presenting a low capacitive load. The xe2x80x9cletting inxe2x80x9d of the piezoelectric insert is a complex process however and therefore renders this technique expensive.
The present invention has as an objective printheads that have a lower capacitive load than known constructions and yet are not complex to produce. Methods of manufacture of such printheads are also comprised in the present invention.
Accordingly, the present invention consists in droplet deposition apparatus comprising
a bottom sheet comprising piezo-electric material and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls and by a bottom surface extending between said side walls;
at least the side walls comprising said piezo-electric material and electrodes for applying an electric field to said piezo-electric material, thereby to effect transverse displacement of said side wall;
a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof;
the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom; each channel having one portion open on a side lying parallel to the channel axis for communication with droplet fluid supply means and another portion which is closed on all sides lying parallel to the channel axis;
characterised in that
the respective side walls comprising said piezo-electric material in said one portion of each channel are disabled such that transverse displacement of said side walls in said one portion of each channel does not take place.
It has been recognised by the present inventors that not only is transverse displacement of the walls in xe2x80x98openxe2x80x99 sections of the channel unnecessary but that by disabling the walls in such xe2x80x98openxe2x80x99 sections, a reduction in the overall capacitative load can be achieved in printheads manufactured according to the substantially conventional technique of forming channels in a base comprising a uniform layer of piezo-electric material. The complex manufacturing methods described above can therefore be avoided.
According to a first preferred aspect, the invention consists in droplet deposition apparatus comprising a bottom sheet having a layer of piezo-electric material poled in a direction normal to said layer and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls and by a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material;
a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof;
the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom;
each channel having one portion open on a side lying parallel to the channel axis for communication with droplet fluid supply means and another portion which is closed on all sides lying parallel to the channel axis;
electrodes provided on opposite sides of said side walls, thereby to form shear mode actuators for effecting droplet expulsion from the channels associated with said actuators, each electrode extending substantially the length of the channel; characterised in that,
in said one portion of said channel a layer of material having a dielectric constant lower than that of the piezoelectric material is interposed between said piezoelectric material and at least one of said electrodes provided on opposite sides of said side walls.
Such apparatus achieves a lower capacitance than conventional designs without the complexity associated with xe2x80x9clet-inxe2x80x9d piezoelectric inserts as described above. In said one portion of the channel which is open to ink supply means, the walls do not need to be displaceable. Consequently the electrodes in such a portion can be separated from the piezoelectric material of the channel walls by a layer of material having a lower dielectric constant than the piezoelectric material thereby disabling that part of the piezo electric material. The resulting capacitive load between the electrodes on opposite sides of the channel wall in this one portion is lower than that obtained with piezoelectric material alone (as is the case in said another xe2x80x9cclosedxe2x80x9d portion of the channel) and thereby contributes to a reduction in the total capacitive load of the printhead.
According to a particular embodiment, said-one and another portions are each defined at least in part by side walls having co-planar top surfaces and being of substantially constant height, whereby the height of the respective side walls of said one and another portions is substantially equal. Such an embodiment is particularly amenable to manufacture, there being no variation in channel machining depth required in the forward part of the channels at least. In such an embodiment, said another portion of the channel may extend at a constant depth as far as the nozzle. Independently of this, said one and another portions may be contiguous.
Preferably the electrodes provided on opposite sides of the side walls are located at the top of each channel side wall, remote from said bottom surface. Such an arrangement is particularly amenable to deposition using the known xe2x80x9cangled platingxe2x80x9d principle. In particular, the electrode may advantageously extend over approximately half the depth of the respective channel wall in said another portion whilst in said one portion, the electrode preferably extends over 10% or less of the height of the respective channel wall.
Where each of said channels comprises a further portion having channel walls of lesser height than the channel walls of either said one or another portions, electrodes being provided on the facing channel walls and the bottom of said further portion of each of said channels, there is provided a layer of material having a dielectric constant lower than that of the piezoelectric material interposed between said electrodes and said facing walls and bottom of each channel. The electrodes in such a further portion serve as connection points between the electrodes in the channels and external driver chips. A layer of low dielectric material between these electrodes and the reduced-height channel walls reduces the capacitive load of these areas.
This aspect of the present invention also consists in a method of making a droplet deposition apparatus including the steps of (a) forming a multiplicity of parallel channels in a bottom sheet comprising a layer of piezo-electric material; (b) depositing a layer of material on at least one of the facing side walls of a first portion of each of said channels, the material having a dielectric constant lower than that of the piezo-electric material, the facing side walls of a second portion of each of said channels remaining free of said material; (c) depositing electrode material on the facing side walls of said first and second portions of said channels. Again, this method allows a low-capacitance printhead to be made without complexity by inventive use of deposition methods as are known per se.
Advantageously, the method involves masking said second portion of each of said channels prior to depositing said layer of material on at least one of the facing side walls of said first portion of each of said channels and then unmasking said second portion prior to depositing electrode material on the facing side walls of said first and second portions of said channels.
The electrode material is advantageously deposited by means of metal vapour beams directed at said walls and at angle to channel facing surfaces thereof.
This first aspect of the invention also covers droplet deposition apparatus comprising a bottom sheet comprising a layer of piezo-electric material poled in a direction normal to said layer and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls having a height and a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material; a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof; the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom; each channel having a portion which is closed on all sides lying parallel to the channel axis; electrodes provided at least on opposite sides of said side walls, thereby to form shear mode actuators for effecting droplet expulsion from the channels associated with said actuators, each electrode extending substantially the length of the channel; characterised in that a layer of material having a dielectric constant lower than that of the piezoelectric material is interposed between said piezoelectric material and at least one of said electrodes in areas other than said portion of each channel.
The first aspect of the invention also includes droplet deposition apparatus having a bottom sheet comprising piezo-electric material and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls having a height and a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material; a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof; the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom; each channel having a portion which is closed on all sides lying parallel to the channel axis; electrodes for applying an electric field to said piezo-electric material in said side walls; characterised in that, in areas other than said portion of each channel, a layer of material having a dielectric constant lower than that of the piezoelectric material is interposed between said piezoelectric material in said side walls and at least one of said electrodes.
Similar advantages are achieved by a second preferred aspect of the present invention. This consists in droplet deposition apparatus comprising a bottom sheet having a layer of piezo-electric material poled in a direction normal to said layer and formed with a multiplicity of parallel, open-topped channels mutually spaced in an array direction normal to the length of the channels, each channel being defined at least in part by facing side walls having co-planar top surfaces and by a bottom surface extending between said side walls, at least the side walls comprising said piezo-electric material; a top sheet facing said bottom surfaces of said channels and bonded to said side walls to close said channels at the tops thereof; the channels being supplied with droplet liquid fluid and communicating with a nozzle for ejection of droplets therefrom; each channel having one portion open on a side lying parallel to the channel axis for communication with droplet fluid supply means and another portion which is closed on all sides lying parallel to the channel axis; electrodes provided on opposite sides of said side walls, thereby to form shear mode actuators for effecting droplet expulsion from the channels associated with said actuators, each electrode extending substantially the length of the channel; characterised in that the electrodes provided in said one portion of each channel extend over a lesser proportion of the height of the respective side walls than do the electrodes provided in said another portion of each channel.
Such apparatus again achieves a lower capacitance than conventional designs in a simple manner. In said one portion of the channel which is open to ink supply means, the walls do not need to be displaceable and consequently the electrodes in such a portion need only extend over a (lesser) proportion of the channel wall height sufficient to convey the actuating voltages to the electrodes in the closed portion of the channel but insufficient to drive the piezo electric material in the open portion, which thus is locally disabled. In the closed portion, the electrodes extend a correspondingly greater proportion down the wall sufficient to cause the channel walls to move. In this way, the capacitive load of the channel walls in said one (open) portion at least is reduced relative to conventional designs, resulting in a reduction in the capacitance of the printhead as a whole.
According to a particular embodiment, said one and another portions are each defined at least in part by side walls of substantially constant height, whereby the height of the respective side walls of said one and another portions is substantially equal. Such an embodiment is particularly amenable to manufacture, there being no variation in channel machining depth required in the forward part of the channels at least. In such an embodiment, said another portion of the channel may extend at a constant depth as far as the nozzle. Independently of this, said one and another portions may be contiguous.
Preferably the electrodes provided on opposite sides of the side walls are located at the top of each channel side wall, remote from said bottom surface. Such an arrangement is particularly amenable to deposition using the known xe2x80x9cangled platingxe2x80x9d principle. In particular, in said one portion, the electrode preferably extends over 10% or less of the height of the respective channel wall.
The invention according to this second aspect is particularly advantageous in embodiments where the ratio of the lengths of said one and another channel portions is approximately 2 or greater.
The present invention also comprises a corresponding method of manufacture which includes the steps of (a) forming in a bottom sheet comprising a layer of piezo-electric material a multiplicity of parallel channels separated by channel walls having co-planar top surfaces, (b) depositing electrode material on the facing side walls of a first portion of each of said channels over a first proportion of the height of the respective side walls; (c) depositing electrode material on the facing side walls of a second portion of each of said channels over a second proportion of the height of the respective side walls, said first and second proportions being different.
Again, this inventive use of per se known deposition methods allows a low-capacitance head to be manufactured without complexity.
According to a preferred embodiment, the method includes the steps of masking said second portion of each of said channels prior to depositing electrode material on the facing walls of said first portion of each of said channels over a first proportion of the height of the respective side walls; and unmasking said second portion prior to depositing electrode material on the facing side walls of said first and second portions of each of said channels. Such a procedure is simple to realise and does not require the breaking of vacuum between deposition steps. Advantageously, the first proportion is greater than said second proportion, the second proportion being preferably no greater than 10% of the wall height. This may be achieved by an inventive use of the conventional technique of material deposition by means of metal vapour beams directed at said walls and at angle to channel facing surfaces thereof, the angle for deposition in said first portion being steeper that the angle for deposition in said second portion.